Currently around 80% of the worldwide energy demand is met by the combustion of fossil fuels, the burning of which gives rise to worldwide annual atmospheric emissions of approximately 34 000 million tonnes of carbon dioxide. This release into the atmosphere is the major contribution of carbon dioxide, which in the case of a lignite power station, for example, can be up to 50 000 tonnes per day. Carbon dioxide is one of the gases known as greenhouse gases, whose negative effects on the atmosphere and the climate are debated. Since carbon dioxide occupies a very low position thermodynamically, it is difficult to reduce it to give reusable products, a fact which has left the actual recycling of carbon dioxide to date within the realm of theory or of academia. Natural breakdown of carbon dioxide is accomplished, for example, by photosynthesis. A replica of the natural photosynthesis process using industrial photocatalysis has to date lacked adequate efficiency.
One alternative is the electrochemical reduction of carbon dioxide. Systematic studies of the electrochemical reduction of carbon dioxide are still a relatively young field of development. Efforts to develop an electrochemical system able to reduce an acceptable volume of carbon dioxide only emerged a few years ago. Laboratory-scale research efforts have shown that, preferentially, metals are to be used as catalysts for the electrolysis of carbon dioxide. While carbon dioxide is reduced almost exclusively to carbon monoxide at silver, gold, zinc, palladium, and gallium cathodes, for example, the reaction products at a copper cathode comprise a multitude of hydrocarbons.
FIG. 1 shows a construction of an electrolysis system according to the prior art. The construction exhibits an electrolysis cell 1 having an anolyte circuit and a catholyte circuit 20 and 21, separated by means for example of an ion exchange membrane in the electrolysis cell. In this case, typically, different electrolytes are used in the anolyte and catholyte circuits. These electrolytes are held in reservoirs 201 and 211, where they are cleaned. A typical construction, shown in simplified form, of an electrolysis system comprises an electrolysis cell having an anolyte circuit and a catholyte circuit. These circuits are separated from one another in the electrolysis cell by means of an ion exchange membrane. The respective electrolyte is held in reservoirs, where it is cleaned.
If the electrolyte used in both circuits is the same, prolonged operation of the electrolysis is accompanied by changes both in the pH and also in the ion concentration in the individual solutions. The membrane additionally complicates the construction. If, for example, the anolyte and catholyte used comprise a 0.5 M KHCO3 solution, the cell voltage after a couple of hours increases sharply, since the cations have migrated from the anolyte chamber into the catholyte chamber to the electrode as a result of the electrical voltage applied. Although the osmotic pressure is compensated to start with, or even counteracted after a certain time, the electrical attraction of the cathode is stronger and the migration of cations proceeds primarily in one direction. If the initial concentration is raised or the anolyte is periodically renewed, crystallization of KHCO3 in the catholyte can be found after a few hours. Similar comments also apply in respect of electrolytes whose electrical conductivity is generated by other salts (sulfates, phosphates).